A Dublin man must wait to hear the outcome of an appeal against a seven-year sentence for killing his partner's son.

David Mahon (46) had pleaded not guilty to the murder of Dean Fitzpatrick (23), the older brother of missing teenager Amy, on May 26, 2013.

Dean Fitzpatrick received a stab wound to the abdomen outside the apartment his mother, Audrey Fitzpatrick, shared with David Mahon at Burnell Square, Northern Cross, on the Malahide Road in Dublin.

The two-week trial heard that Mahon had been in a relationship with Audrey Fitzpatrick for 12 years by the time her son died.

The State had argued that Mahon was drunk, angry and agitated when he thrust a knife into his stepson with deadly intent. Mahon claimed his death was an accident or possible suicide and that Mr Fitzpatrick had ‘walked into the knife’ while they had been arguing.

A Central Criminal Court jury found Mahon not guilty of murder but guilty of manslaughter and he was sentenced to seven years imprisonment by Ms Justice Margaret Heneghan on June 13, 2016.

Opening an appeal against the severity of his sentence today, Mahon's barrister, Seán Guerin SC, said there was no case “quite like this”.

He said Mahon was factually proven only to have produced the knife. That was all Mahon did, Mr Guerin said. He produced the knife, he didn't thrust it, he didn't stab, and because he didn't thrust it, he didn't commit an act that could have amounted to a physical assault.

Mr Guerin there were two distinct views open to the jury. The prosecution case for murder was that Mahon took out a knife and deliberately stabbed Mr Fitzpatrick causing his death.

The alternative view was that Mahon had merely produced the knife as if to show the victim “as it were”. He either intended to put him in fear or was reckless to put him in fear.

Mr Guerin said the only conclusion which could be drawn from the verdict is that the jury were not satisfied beyond a reasonable doubt that Mahon had stabbed the deceased.

Once that is acknowledged, Mr Guerin said it becomes clear that this was a “very unusual case” involving a “very low” level of culpability.

Mr Guerin said the sentencing judge erred in identifying a number of aggravating factors.

He said the inherent gravity of the crime was not an aggravating factor, “it's the essence of the offence”.

The fact Mahon had a knife in his possession was true but he took the knife from Mr Fitzpatrick having disarmed him, Mr Guerin said.

He said the disposal of the knife didn't reflect well on Mahon and it was “something he shouldn't have done” but the prosecution were at no significant forensic disadvantage by its loss, counsel said.

Mr Guerin said there was a knife block in the kitchen with one empty slot out of six. It would have been the “easiest thing in the world” to ask the manufacturer in Sheffield what the sixth knife looked like.

Within 12 hours of the incident, Mahon called the gardaí to say he was responsible. Within twenty minutes of that phone call, he was in the garda station answering questions and he fully co-operated with the investigation.

Mr Guerin submitted that the offence was at the lower end of the range for manslaughter cases.

Counsel for the Director of Public Prosecutions, Anne-Marie Lawlor SC, said the entire submission from Mahon's lawyers was “predicated on a fallacy”. Twice in their written submissions, she said Mahon's lawyers referred to a “successful defence of the case”.

Ms Lawlor said the word manslaughter was not used once by the defence in their closing speech to the jury and it was never suggested that this was an involuntary manslaughter and that that was an available verdict.

She said the defence case was that their had been an accidental self impalement.

Ms Lawlor said the verdict left no room for ambiguity. The jury rejected the notion of accident and rejected the “somewhat fanciful notion” of self impalement.

She said Mr Guerin had not addressed the fact Mahon committed the offence while being subject of a suspended sentence for a minor offence.

She said it was “bizarre” to suggest the gardaí could have contacted the manufacturer of the knife for an alternative weapon.

She said Mahon callously disposed of the weapon “one can only summise to ensure it wouldn't be available” to the investigation.

Mr Justice Michael Peart, who sat with Mr Justice George Birmingham and Mr Justice Alan Mahon, said the court would reserve judgment.